1. Field of the Invention
This invention relates to a sheet transport apparatus for use in a printer, a facsimile machine and, more particularly, an ink jet type image forming apparatus.
2. Description of the Related Art
An ordinary, conventional ink jet recording apparatus will be described with reference to FIGS. 12 and 13. FIG. 12 shows a recording head 101 for recording by ejecting ink, a carriage 102 capable of moving while supporting the recording head 101, guide rails 103 for supporting and guiding the carriage 102, a motor 104 for driving the carriage 102, a pulley 105 directly connected to the motor 104, a follower pulley 106 opposite to the pulley 105, a wire 107 wrapped around the pulley 105 and the follower pulley 106 to transmit motive power of the motor 104 to the carriage 102, a recording medium 109, such as paper, a paper feed motor 110 for moving the recording medium 109, a cap 112 for protecting nozzles from drying and other problems, a transport roller 115 for transporting the recording medium 109, a pressing roller 116 for pressing the recording medium 109 against the roller 115 by using an urging means (not shown), and a non-recording ejection box 117 positioned between the cap 112 and the recording medium 109 and used for receiving ejection of ink droplets from the recording head 101 other than ejection for recording. The carriage 102 is movable in the directions of arrows 113, and the roller 115 is rotated in the direction of arrow 114.
When recording is performed by this apparatus, the recording head 101, having the nozzles protected by the cap 112, is moved away from the cap 112, and the motive power from the motor 104 is transmitted to the wire 107 wrapped around the pulley 105 and the follower pulley 106. The recording head 101 is thereby moved together with the carriage 103 parallel to the recording medium 109 and is moved through a predetermined range in the vicinity of the recording medium 109 to scan the same. Thereafter, the direction of movement of the recording head 101 is reversed and the recording head 101 is moved toward the cap 112. During this scan, the recording head 101 ejects ink droplets at predetermined positions to perform recording while traveling back and forth in the directions of arrows 113. Each time this cycle of scanning of the recording head 101 on the recording medium 109 is completed, the recording medium 109 is fed through a predetermined distance along the direction of arrow 114 by the paper feed motor 110 and the roller 115. These operations are repeated to perform recording.
However, the above-described conventional recording apparatus has drawbacks described below. In the example of the conventional apparatus shown in FIG. 12, recording cannot be performed on an end portion of the recording medium 109 located between the nip of the transport and pressing rollers 115 and 116 and the nozzle at the end of the head closer to the pressing roller 116 by a final scanning stroke, because there is no means for accurately feeding this portion of the recording medium 109 to the recording position. Therefore, a margin of a recorded page corresponding to such a trailing end portion of the recording medium 109 is large and a suitable image size cannot be obtained.
FIG. 13 illustrates the size of the margin. In particular, if the recording medium 109 has a certain large size such as A1 or A0 size, then the pressing roller 116, the length of which is correspondingly large, must have an increased diameter in order to maintain its desired strength, resulting in a further increase in the size of the margin. If a pair of rollers are provided on the downstream side of the recording medium 109, a similar margin is formed at the leading end of the recording medium 109.
A recording apparatus having a similar construction and having pairs of rollers respectively provided on the upstream and downstream sides of the recording medium 109 facing the recording head 101 is also known. In this apparatus, if the feed rates of the pairs of rollers are equal to each other, there is a possibility of the recording medium bending between the pairs of rollers to contact the recording head when feed errors are accumulated. To prevent occurrence of such a phenomenon, the feed rate of the downstream rollers is set so as to be a significant percentage larger than that of the upstream rollers. In this arrangement, however, the recording medium 109 is fed only by the downstream rollers after the trailing end of the recording medium 109 has passed the upstream rollers. Unless the feed rate of the downstream rollers is changed at this time, the accuracy of image formation is reduced. A complicated feed rate control is therefore required for this arrangement.
Water is ordinarily used as a main solvent for recording ink. If an image is recorded at a high density, a large amount of water is applied to the recording medium 109 to permeate into the same, thereby causing the recording medium 109 to swell and increase in size. As a result, a cockling or warping phenomenon occurs such that the recording medium 109 is cockled in the recording area. If the height of cockles thereby formed is increased, it is possible that the recording medium 109 will contact the head 101 to cause a disturbance in the resulting image or that clogging will occur in the nozzles of the recording head 101.